Combined filtration and fixation of heavy metals

ABSTRACT

The present invention describes a filter medium and a method of filtering metals from liquids, such as waste water, in which the metals are filtered from the liquids and are chemically fixed in the resulting filter cake in a nonhazardous and nontoxic form so that they may be disposed of in nonhazardous landfills. When all or a portion of the metals are dissolved, they are first precipitated and then filtered and chemically fixed.

FIELD OF THE INVENTION

The present invention relates to filtering and chemically fixing in thefilter medium hazardous heavy metals in liquids and particularly inwaste waters.

BACKGROUND OF THE INVENTION

Large quantities of hazardous metal-contaminated waste water and otherliquids have been discharged in the environment without treatment.Current federal and state regulations limit the hazardous metalconcentrations in waste water and are extremely severe and arefrequently based on analytical detection limits. Most metals are presentin the waste water at concentrations which can range from 10 to 4,000parts per million. Under some current regulations, all hazardous metalconcentrations in waste water are required to be less than 300 parts perbillion and some to less than 20 parts per billion. The hazardous metalsinclude cadmium, chromium, copper, lead, manganese, selenium, as well asothers. In addition, it is desirable to remove and fix all metals, someof which are currently considered to be nonhazardous, such as zinc.These hazardous metals, as well as nonhazardous metals, are frequentlydissolved in waste water and, in order to remove them, it is necessaryfirst to precipitate them. This is accomplished by any number of knowntechnologies, for example, hydroxide precipitation, chemical oxidation,insoluble salt formation and the like. Metal hydroxide precipitation isperhaps the most common at the present time. While metal hydroxideprecipitation is a widely known and extensively used method for removingmetals from waters, the resulting slurry from metal hydroxideprecipitation has been difficult to filter, and the filter cake ishazardous and will not pass regulatory tests for hazardouscharacteristics. It is highly desirable to provide for the removal ofheavy metals from waste waters in which the dissolved metals areprecipitated, the filter ability of the resulting slurry of precipitatedmetals is improved, and the hazardous metals are chemically fixed in thefilter cake which is readily removable and which filter cake isnonhazardous and does not need to be disposed in a hazardous waste site.

SUMMARY OF THE INVENTION

The present invention is directed to such a filter medium for and tomethods of filtering and chemically fixing hazardous dissolved metal,hazardous and nonhazardous, in liquids, such as waste waters. Thedissolved metals are precipitated using any number of knowntechnologies, such as hydroxide precipitation, chemical oxidation,insoluble salt formation and the like. Metal hydroxide precipitation isthe most common and is preferred. The filter medium comprises siliciousparticles and at least one polyvalent metal ion, the silicious particlesbeing present in an amount effective to filter the dissolved metalprecipitates from and to form a soluble silicate in the liquid, thepolyvalent metal ion being present in an amount sufficient to form asilicious cement with the soluble silicate and to chemically fix themetal particulates in the silicious cement. Accordingly, it is an objectof the present invention to provide a filter medium in whichprecipitated dissolved metals are filtered from liquids such as wastewaters and are chemically fixed in the filter medium, and the filtermedium having the chemically fixed metals is nontoxic and nonhazardous.

A further object of the present invention is the provision of a methodof filtration and chemically fixing metals precipitated in liquids, suchas waste waters and recovering them in nontoxic or nonhazardous form.

A further object of the present invention is the provision of filteringliquids having precipitated dissolved metals from the liquids utilizingas the filter medium silicious particles and a polyvalent metal ioneffective to dissolve at least a portion of the silicious particles toform a soluble silicate and sufficient to form a silicious cementitiousproduct with the polyvalent metal ion effective to solidify andchemically fix precipitated dissolved metals.

Other and further objects, features, and advantages appear throughoutthe specification and claims.

DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS

As previously mentioned, the present invention is directed to a meansfor and a method of filtering precipitated dissolved metals fromliquids, such as waste waters utilizing silicious particles which filterall of the metal particulates, the silicious particles forming a solublesilicate from the liquids, and in the presence of a polyvalent metal ionforms a silicious cement chemically fixing the precipitated metalparticles.

The filter medium for filtering and chemically fixing precipitateddissolved metals in liquids, such as waste water, includes siliciousparticles and at least one polyvalent metal ion, the silicious particlesare present in an amount sufficient to filter the metal precipitates andform a sufficient amount of soluble silicate in the liquid for thecement reaction, and the polyvalent metal ion is present in an amountsufficient to form a silicious cement with the silica particles andchemically fix the metal particulates in the silicious cement. If thepolyvalent metal ion is present in such an amount in the waste water, itmay be omitted from the filter medium.

The method of filtering and chemically fixing of precipitated dissolvedmetals in liquids, such as waste waters, comprises flowing the liquidthrough a filter comprised of a mixture of silicious particles in thepresence of a polyvalent metallic ion, the silicious particles beingeffective to filter the metal particulates from the liquid and containthem, the liquid having a pH in a range sufficient to form a solublesilicate with the silicious particles but not to dissolve the metalprecipitates, the polyvalent metal ion being present in the filter orwaste water or both in an amount effective to form a silicious cementwith the silicious particles and chemically fix and contain the metalprecipitates.

The method of filtering and chemically fixing the metals in waste watersincludes adding the silicious particles and polyvalent metallic ion tothe waste waters after the pH has been adjusted, if necessary, to form aslurry which is then filtered which produces a cementitious filter cakewith the metals chemically fixed and contained in the filter cakewhereby the solids are captured in the filter and the liquid passesthrough. Both are now nonhazardous and the liquid has had the metalsremoved and the solids pass regulatory testing for hazardous wastecharacteristics.

The silicious particles preferably are amorphous, although crystallineparticles may be utilized, but they tend to slow down the reactionperiod. The silicious particles preferably are biogenetic silicaparticles, such as those produced by pyrolysis or burning of plants andhulls containing large amounts of silica, that is, having a minimum of15 percent silica by weight of the dry matter and preferably 20 percentor more which leaves an ash that is high in silica. If desired, a smallamount of carbon uniformly dispersed throughout the silica structure maybe included. The presently preferred biogenetic silica is rice hull ash.Other silicious particles, such as diatomaceous earth and perlite may beutilized. The silicious particles used should be capable of forming asoluble silicate in a caustic liquid or waste water.

The presently preferred polyvalent metal ion is Portland Cement (PC)because of its availability and price. Any polyvalent metal ion can beused which will react to form the silicious cement with the formedsoluble silicate; for example, calcium oxide (quick lime), coal fly ash,potassium oxide, aluminum sulfate, alumina chlorohydrate and the like.Thus, any source of polyvalent metal ion can be used. In some cases, thepreferred source may be one with limited solubility in waste so that themetal ion is released slowly over a long period of time; and, in othercases, it is acceptable to have the metal ion entirely in solution whenthe mixture is made. The alkali and the polyvalent metal may be ineither solid or liquid form.

The relative proportions of the ingredients may vary from waste towaste.

As previously mentioned, of the biogenetic silica, rice hull ash ispreferred although plants that contain 15 percent or more silica byweight in its dry matter are satisfactory, for example, stalks and hullsof rice, equisetum (horsetail weeds), certain bamboos and palm leaves,particularly polymra, pollen and the like, all of which when burnedleave a porous ash that is highly desirable as a filtration medium.Preferably, the silicious particles should be amorphous althoughcrystalline particles may be present. The only disadvantage tocrystalline particles is that they slow down the reaction considerably.

It is important that all of the dissolved free metal ions beprecipitated in metal hydroxide form so that they can be completelyreacted with the silicious particles and the polyvalent metal ion.Therefore, the degree of alkalinity is controlled by the solubility ofmetal hydroxides being treated. The theoretical goal is to adjust thelevel of alkalinity so that the metal hydroxides are the least soluble.At this point of low insolubility it is most likely that the greatestamount of the free metal ions will have reacted to become metalhydroxide. This in turn insures a complete reaction with the siliciousparticles and the polyvalent metal ion. Thus, the dissolved metals willbe precipitated and removed from the water and made reaction productsfixed in the resulting silicious cement.

The presently preferred biogenetic silica is rice hull ash. Rice hullsare high in silica content, containing about 18 to 22 percent by weight,with the ash having a porous skeletal silica structure havingapproximately 75 to 80 percent open or void spaces by volume. Inaddition, it has been a continuing problem for the rice industry todispose of rice hulls; and, while a number and variety of uses for ricehulls or rice hull ash have been proposed ad used, large volumes of ricehulls are burned; and their ash is disposed by the rice industry as awaste material at great expense.

Biogenetic silica in amorphous state and in substantially porous formcan be obtained either by burning or decomposition of the hulls. Anyprocess can be used to obtain the ash, preferably, high in amorphoussilica.

As an example, commercially available rice hull ash can be used and isprepared by burning rice hulls in a furnace. In the process, raw ricehulls are continually added to the top of the furnace and the ash iscontinuously removed from the bottom. Temperatures in the furnace rangefrom 800° to about 1400° C., and the time factor for the ash in thefurnace is about three minutes. Upon leaving the furnace, the ash israpidly cooled to provide ease in handling. When treated by this method,silica remains in a relatively pure amorphous state rather than thecrystalline forms known as tridymite or crystobalite. The transitionfrom the amorphous to the crystalline state generally takes place whenthe silica is held at very high temperatures, for example 2000° C. orlonger periods of time. The significance of having the silica in anamorphous state is that the silica ash maintains a porous skeletalstructure rather than migrating to form crystals, and the amorphous formof silica does not cause silicosis thus reducing cautionary handlingprocedures. The burning of the rice hulls is time-temperature related,and burning of these hulls under other conditions can be done so long asthe ash is in an amorphous state with a porous skeletal structure.

The amount of open or void spaces in the biogenetic silica ash dependson the amount of fines in the ash. The inclusion of fines is notdeleterious; however, the more porous the ash the better.

On a commercial burning of rice hulls as an energy source, the resultantash had the following chemical analysis (by weight):

    ______________________________________                                        Silicon           93.0-94.0 percent                                           Carbon             5.0-5.5 percent                                            Balance           trace minerals                                              ______________________________________                                    

The trace minerals consist of minor amounts of magnesium, barium,potassium, iron, aluminum, calcium, copper, nickel, and sodium.

The carbon content is in a dispersed state throughout the material. Ifdesired, the carbon can be actuated for treating with super heated steamunder standard conditions. This treatment removes particles that clogthe pores of the carbon thus enormously increasing its filter ability.

As previously mentioned, the ash, rice hull ash or other agriash can beused satisfactorily irrespective of how it is obtained, but preferablyshould be predominantly amorphous.

In practice, a sample of the waste water containing the dissolved metalto be filtered and chemically fixed in a nonhazardous filter cake isobtained and tested.

Since it is important that all of the free metal ions in the waste waterbe precipitated or in metal hydroxide form so that they will becompletely reacted with the silicious particles and the polyvalent metalions, such as Portland cement, and the degree of alkalinity ascontrolled by the solubility of metal hydroxide being treated, the levelof alkalinity preferably is adjusted so that the metal hydroxides areleast soluble. At this point of low insolubility, it is most likely thatthe greatest amount of the free metal ions will have reacted to becomemetal hydroxide precipitates. This, in turn, insures a complete reactionwith the silicious particles and the polyvalent metal ions. This resultsin the precipitated metals being removed from the water and madereaction products which are chemically fixed in the filter cake. Aspreviously mentioned, metal hydroxide precipitation is a widely knownand extensively used method for removing metals from waters. However,the slurry resulting from metal hydroxide precipitation is traditionallydifficult to filter, and the filter cake will not pass regulatory testsfor hazardous characteristics. Advantageously, treatment of metals inwaste water yields a filter cake which passes regulatory testing forhazardous characteristics and, unexpectedly, improves filterability ofthe slurry.

Preferably, the first step is to optimize the hydroxide treatment. Thisoptimization focuses on complete reaction of all of the free metal ionswith the alkaline source and yields a comparison of free metal ioncontent in the water to pH for several alkaline sources, for example,calcium hydroxide, sodium hydroxide, potassium hydroxide, and the like.This screening will determine which alkaline source and pH is mosteffective.

The next step is to prepare the waste water at its proper alkalinity foroptimal metal hydroxide formation by adding the silicious particles andpolyvalent metal ions at varying levels and testing for filtrationcharacteristics. In many cases, the amount of silicious particles andpolyvalent metal ions necessary for filtration organization providesenough to satisfy the chemical requirements for fixation. For example,when utilizing 75 percent rice hull ash and 25 percent Type 1 Portlandcement, the amount of silicious particles and polyvalent metal ionsshould be approximately five times that of all cations present in thewater but no less than 2,000 parts per million (ppm). The five timesratio can be reduced for total cation concentrations in the water, forexample, above 5,000 ppm.

In the case of where the requirement for filtration enhancement is farmore than that required for chemical treatment, charged polyelectrolytepolymer flocculation can be used. The advantage is that the polymerworks in concert with the silicious particles to enhance filtrationcharacteristics and results in a far lower amount of silicious particlesand polyvalent metal ions which lowers the treatment and disposal costs.The chemical properties of the silicious particles and polyvalent metalions are not affected by this treatment as this is an extension of aproperty of silicious particles, such as rice hull ash which providemassive charge sites for charge polymers to seed from. This is extremelyeffective in several forms of filtration and clarifier technologies. Thefollowing examples are illustrative of the invention.

EXAMPLE 1

A product composed of a mixture of 75 percent rice hull ash (RHA) and 25percent Type 1 Portland cement (PC) was used in a demonstration forUnited States Environmental Protection Agency's Superfund InnovativeTechnology Evaluation (SITE) program. In this program the EPA matchesthe application technology to an existing field problem from theSuperfund listed sites. In the present example, the EPA providedPalmerton Zinc Superfund Site which is a ground water contaminatedprimarily with zinc. Cadmium and lead were also present but in lowconcentrations. The goal of the project was to demonstrate that theprocess and product could treat the ground water from about 450 ppm zincto less than 1.5 ppm at commercial flow rates and yield a filter cakewhich would pass regulatory testing, that is be nonhazardous and may bedisposed of in nonhazardous landfills.

In the first steps of the evaluation procedure the alkalinity source andamount were screened. Caustic soda (sodium hydroxide NaOH), and lime(calcium hydroxide, Ca(OH)₂) were evaluated. Lime was selected on costand availability criteria. The screening data is reported in Table 1.

                  TABLE 1                                                         ______________________________________                                        Alkaline Source, pH and Zn Removal                                            Alkaline Source pH     Zinc in Filtrate (ppm)                                 ______________________________________                                        Sodium Hydroxide                                                                              9.1    0.49                                                   Calcium Hydroxide                                                                             9.8    ND                                                     Calcium Hydroxide                                                                             10.6   1.13                                                   Calcium Hydroxide                                                                             11.6   12.1                                                   ______________________________________                                    

The data in Table 1 indicates that the most effective range for totaldissolved metal precipitation is 9.5 to 10.5. This corresponds topublished chemical data.

The next step was to evaluate the product on filtration performancecriteria. These data are presented in Table 2.

                  TABLE 2                                                         ______________________________________                                        Product Amount vs Filter Flux                                                 Product Amount Gram/Liter                                                                       Filter Flux (gpm/ft.sup.2)*                                 ______________________________________                                        5.9               0.55                                                        11.1              0.74                                                        20.0              1.32                                                        ______________________________________                                         *gpm/ft.sup.2 : gallon per minute per square foot of filter area         

The final step in the evaluation is to confirm that the product wasfixing the metals in the filter cake. These data are reported in Table3.

                  TABLE 3                                                         ______________________________________                                                        EP Tox*                                                                       Filter Cake Leachate (ppm)                                    Product Amount (gram/Liter)                                                                     Zinc     Cadmium    Lead                                    ______________________________________                                        0                 6630     0.73       ND                                      11.1, Tested at 10 Days                                                                         1.5      0.06       0.26                                    11.1, Tested at 30 Days                                                                         1.75     0.06       0.20                                    15.0, Tested at 10 Days                                                                         0.3      0.04       0.31                                    15.0, Tested at 30 Days                                                                         0.13     0.10       0.19                                    ______________________________________                                         *EP Toxicity test, 40 CFR 261                                            

It is significant as shown in Table 3 that the unmistakable trend is forthe metal concentrations in the leachate from the filter cake todecrease over time. This trend will continue forever practicallyspeaking.

The foregoing data confirm the mixture of 75 percent rice hull ash and25 percent Type 1 Portland cement, provides improved filtrationperformance, and fixes the metals in the filter cake to render itnonhazardous.

EXAMPLE 2

In this example the problem consisted of a plant waste water that neededto be prepared for discharge into a costal area. Regulatory limits forthis project are exceedingly low: 13 ppb Copper, 294 ppb Zinc, 56 ppbLead, 227 ppb Chromium, and 132 ppb Nickel. The concentrations in thewater prior to treatment were 2600 ppb Copper, 592 ppb Zinc, 413 ppbNickel, 90 ppb Lead, and 434 ppb Chromium.

A product composed of 75 percent RHA, 25 percent PC and 10 parts permillion polymer composed of highly charged, cationic high molecularweight polyacrylamide was used.

In this example there was no pH adjustment required for two reasons.First, the natural pH of the waste water ranged from 8.5 to 10.5, and itwas determined that all of the metals were already in metal hydroxideform and there were little or no dissolved metals.

The testing then was directed to removing the suspended metals from thewater, maximizing filtration flow rates and ensuring that the treatedmetals would not leach out of the filter cake.

The response to metals removal efficiency and flow rates versus theamount of the mixture of RHA and PE were determined at the same time.These data are reported in Table 4.

                  TABLE 4                                                         ______________________________________                                        Flux Metals Removal Efficient vs Product Amount                                                Filter Metals in Filtrate                                                     Flux   (ppb)                                                 RHA and PC Amount (gram/liter)                                                                   (gpm/ft.sup.2)                                                                         Cu     Zn  Ni                                     ______________________________________                                         0*                3.44     171    78  66                                     1                  1.48     ND     52  ND                                     2                  1.84     ND     91  ND                                     3                  3.83     ND     10  ND                                     4                  4.22      30     6  ND                                     ______________________________________                                         *An inert coarse filter aid was used at 0.5 gm/L.                             ND = Not Detected. Detection Limits = 1 ppb Cu, 50 ppb Zn, and 100 ppb Ni                                                                              

These data confirm that the mixture of RHA and PC was effective atremoving the metals from the water and that it also provided excellentfilter aid performance.

The next step was to evaluate the ability of the mixture to fix themetals in the filter cake so that they would pass regulatorylimitations. The data are reported in Table 5.

                  TABLE 5                                                         ______________________________________                                        Metals in Filter Cake Leachate vs Product Amount                                                Filter Cake TCLP* (ppb)                                     Product Amount (gram/Liter)                                                                       Cu     Zn        Ni                                       ______________________________________                                        0                   640    350       770                                      2, Tested at 10 Days                                                                              140    1700      130                                      2, Tested at 30 Days                                                                              ND      4         14                                      3, Tested at 10 Days                                                                              390    460       110                                      3, Tested at 30 Days                                                                               5      26        64                                      ______________________________________                                         *TCLP: Toxic Characteristic Leaching Procedure based on EPA recommended       test (Fed Register 53 (159), Aug. 17, 1988 + EPA CFR Title 40, part 268) 

Again, it is important to note that leachable metals concentration getslower with time.

The amounts of the silicious particles and polyvalent metal ions can bevaried considerably to accommodate the properties of the waste water inwhich the metals are filtered from and fixed in the filter cake in anonhazardous manner. For example, the amount of the silicious particlescan vary from 50 to 85 percent and the amount of polyvalent metal ionscan vary from 15 to 50 percent, by weight. The amount of the filtrationfixation mixture to be added to the waste water can be varied accordingto requirement of the waste water. This can vary from 1 to 100 grams perliter of waste water.

EXAMPLE 3

In this example, other biogenetic silica particles were substituted forrice hull ash in the preceding examples. These include the ash fromstalks and hulls of rice, equisetum, bamboos and pollen leaves whichwhen burned provide a porous ash that is highly desirable as afiltration medium. Satisfactory results were obtained by suchsubstitution.

EXAMPLE 4

In this example, diatomaceous earth and perlite are substituted for therice hull ash of the preceding examples and each provide satisfactoryresults. That is, the filtration of the precipitated metals was obtainedand they were chemically fixed in the filter cake which was nonhazardousand did not need to be disposed of in a hazardous waste site.

EXAMPLE 5

In this example, calcium oxide and coal fly ash are each substituted forPortland cement in the preceding examples and each provide satisfactoryresults, that is filtration of the metals from the waste waters andcontainment of them in the filter cake which was nonhazardous.

EXAMPLE 6

In this example, the various components were added to the waste streamto form a slurry of silicious particles, polyvalent metal ions and a pHto precipitate the metals, and then the slurry was passed through afilter. A cementitious filter cake resulted in which the metals wereremoved from the waste stream and chemically fixed and which wasnonhazardous.

The present invention therefore is well suited and adapted to attain theobjects and ends mentioned as well as others inherent therein.

While presently preferred embodiments of the invention have been givenfor the purposes of disclosure, changes may be made therein and otherapplications may be made thereof which are within the spirit of theinvention as defined by the scope of the appended claims.

What is claimed is:
 1. A method of filtering and chemically fixing metalprecipitates in a liquid comprising,flowing the liquid through a filtermedium comprised of a mixture of silicious particles and in the presenceof a polyvalent metal ion, the silicious particles filtering the metalprecipitates from the liquid and containing them, the liquid having a pHin a range sufficient to form a soluble silicate with the siliciousparticles but not sufficient to dissolve the metal precipitates, therebyforming a soluble silicate by dissolving some of said siliciousparticles; the polyvalent metal ions being present in an amounteffective to form a silicious cement with said soluble silicate, therebyforming said silicious cement and chemically fixing said metalprecipitates in said silicious cement, and wherein said liquid which hasflowed through said filter medium is substantially free of said metalprecipitates.
 2. The method of claim 1 where,the silicious particles arebiogenetic silica ash.
 3. The method of claim 2 where,the filter mediumincludes at least a portion of the polyvalent metal ion.
 4. The methodof claim 1 where,the silicious particles are rice hull ash.
 5. Themethod of claim 4 where,the filter medium includes at least a portion ofthe polyvalent metal ion.
 6. The method of claim 1 where,the siliciousparticles are diatomaceous earth.
 7. The method of claim 6 where,thefilter medium includes at least a portion of the polyvalent metal ion.8. The method of claim 1 where,the silicious particles are perlite. 9.The method of claim 8 where,the filter medium includes at least aportion of the polyvalent metal ion.
 10. The method of claim 1 where,thefilter medium includes at least a portion of the polyvalent metal ion.11. A method of filtering and chemically fixing dissolved metals inwaste water comprising,adding a metal hydroxide to the waste water in anamount sufficient to form metal hydroxide precipitates, flowing thewaste water with the metal hydroxide precipitates through a filtermedium comprised of a mixture of silicious particles and in the presenceof polyvalent metal ions, the silicious particles filtering thehydroxide metal precipitates and containing them, the waste water havinga pH in a range sufficient to form a soluble silicate with the siliciousparticles but not sufficient to dissolve the hydroxide metalprecipitates, thereby forming a soluble silicate by dissolving some ofsaid silicious particles, the polyvalent metal ions being present in anamount effective to form a silicious cement with said soluble silicate,thereby forming said silicious cement and chemically fixing thehydroxide metal precipitates in said silicious cement, and wherein saidliquid which has flowed through said filter medium is substantially freeof said metal precipitates.
 12. The method of claim 11 where,thesilicious particles are biogenetic silica ash.
 13. The method of claim12 where,the filter medium includes at least a portion of the polyvalentmetal ions.
 14. The method of claim 11 where,the silicious particles arerice hull ash.
 15. The method of claim 14 where,the filter mediumincludes at least a portion of the polyvalent metal ions.
 16. The methodof claim 11 where,the silicious particles are diatomaceous earth. 17.The method of claim 16 where,the filter medium includes at least aportion of the polyvalent metal ions.
 18. The method of claim 11where,the silicious particles are perlite.
 19. The method of claim 18where,the filter medium includes at least a portion of the polyvalentmetal ions.
 20. The method of claim 11 where,the filter medium includesat least a portion of the polyvalent metal ions.